The present invention relates generally to lasers and in particular to tunable lasers used in telecommunications systems.
Lasers are widely used in high speed data communication devices such as multi-wavelength fiber optic communication links. In wavelength division multiplexed (WDM) communication links, the transmission of data via an optical fiber is increased by using multiple optical signals with different wavelengths simultaneously. Each optical signal is capable of carrying a different data signal.
Conventional optical sources for such links are often fixed wavelength distributed feedback (DFB) lasers. DFB lasers contain a waveguiding structure fabricated in an active semiconductor, where a continuous grating runs through the device and determines the wavelength of operation. In general these devices are high power, have excellent single wavelength characteristics (side-mode suppression ratio), and are extremely stable over time.
DFB lasers, however, generally operate at fixed wavelengths, and are very difficult to tune to other wavelengths. Though slight changes in wavelength can be realized via thermal effects, tuning DFB lasers by large amounts, to cover a large part of a communication band is often not possible. As such, in order to provide a large, gradual and varied tuning range, generally a large number of DFB lasers fixed at different wavelengths are stocked, along with multiple spare DFB lasers. However, system reconfiguration becomes more complex in order to accommodate multiple DFB lasers.
To overcome these shortcomings, a great deal of effort has been expended on fabricating tunable laser diodes. Unfortunately, delivering the performance of fixed wavelength DFB lasers with the added benefit of wide tunability at an appropriate cost is not easily accomplished. There are fundamental reasons why tunability is such a challenge. In lasers with precise wavelength control, an optical element governs the wavelength, and to tune such a device, the optical path length of the controlling optical element is varied. Varying the optical path length of the controlling element significantly induces other complexities that degrade the performance of the laser.